The present invention relates to an optical information medium capable of recording optically readable information; and, more particularly, to an optimum optical information recording medium adapted for use in recording and reproducing high-density optical information by employing a red laser beam having short wavelengths in the range from 630 to 670 nm.
A digital video disc (DVD) capable of recording and reproducing high-density information has been put into practical use by the help of the recent development and practical utilization of a laser having a short wavelength. In DVD, there are provided an information recording area on at least one main surface thereof, a plurality of pits retaining recorded information and formed on the information recording area, and a reflective layer formed of a metal film and provided on the whole information recording area.
In order to implement the recording and reproduction of high-density information in DVD, DVD standards are established, which are different from those of the compact disc (CD), currently the most widely used optical information recording medium. For instance, according to the standards of the DVD, one DVD disc is required to have a maximum storage capacity of about 4.7 GB, big enough to record video and audio information for an average running time of about 133 min.
Since, however, the current DVD standards are specified only for a read-only DVD such as a DVD-Video storing prerecorded video information and a DVD-ROM storing prerecorded computer program or data, there exist pressing needs to develop a recordable optical information medium having a maximum storage capacity of about 4.7 GB and a recording technique therefor.
The DVD described above has a track pitch of 0.74 xcexcm and a minimum pit size of 0.4 xcexcm (or 0.44 xcexcm in the case of a DVD having dual recording layers); and, therefore, it is possible to achieve higher density recording in DVD than in CD having a track pitch of 1.6 xcexcm and a minimum pit size of 0.83 xcexcm.
A recordable DVD is typically provided with a light transmitting substrate having thereon a tracking groove arranged to have a tracking pitch of 0.74 xcexcm as described above, a recording layer made of a material such as an organic dye formed on the substrate, and a reflective layer made of, e.g., Au or Al formed on the recording layer. Optical information signals can be recorded on such a DVD by forming pits on the light transmitting substrate by irradiating a recording laser beam onto the recording layer. To this end, accurate tracking servo should be ensured by way of, e.g., employing a single wavelength red laser beam within the range of 630-670 nm focused into an extremely small spot. A phase difference method can be used for the tracking servo. However, an access using a phase difference tracking error signal and a radial contrast signal becomes unstable when the radial contrast signal is small. The access described herein indicates a track jump.
In view of the foregoing drawback of the conventional high-density optical information recording medium, a primary object of the present invention is to provide an optical information medium capable of recording optically readable high-density signals in comparison to CD while securing high radial contrast signals to allow stable access.
Another object of the present invention is to provide an optical information medium capable of forming desired pits thereon, entailing reduced jitters while recording high-density signals.
To achieve the above objects, the width D of the top surface of a land is set to be within the range of about 0.30xe2x89xa6Dxe2x89xa60.45 to obtain high radial contrasts in accordance with the present invention. Improved radial contrasts can thus be obtained. A radial contrast represents a value obtained by normalizing the difference between the average levels of the high frequency (HF) signals from a land and a pit after recording by the mean value thereof.
In accordance with the present invention, there is provided an optical information recording medium for recording optically readable signals thereon through the use of a recording laser beam, comprising: a light transmitting substrate; a tracking groove of a spiral shape formed on a surface of the light transmitting substrate; a land formed between the spiral groove; a recording layer formed on the surface of the light transmitting substrate on which the groove and the land are provided; and a reflective layer, formed on the recording layer, for reflecting the recording laser beam, wherein optically readable signals are recorded by the recording laser beam provided through the light transmitting substrate and a width D of the top surface of the land is within the range of about 0.30xe2x89xa6Dxe2x89xa60.45.
The width D described above represents a length of a flat portion of the top surface of the land measured along the radial direction, and, more specifically, a radial length of a portion of the land parallel to the plane of incidence of the recording laser beam.
If D described is within the range of about 0.30xe2x89xa6Dxe2x89xa60.45, large radial contrasts can be obtained from the recorded optical information recording medium and, therefore, accurate tracking can be carried out during the reproduction of the recorded information. On the other hand, if D is less than 0.3, radial contrasts from the recorded optical information recording medium may not be big enough to properly reproduce the recorded optical information. And also, if D is greater than 0.45, the width of the groove of the optical information recording medium may not be big enough to secure proper recording and reproduction of the optical signals.
In addition, it is preferable that a dimension of a protrusion, which can be formed on a peripheral region of the top surface of the land during an injection molding process of the light transmitting substrate, is less than about 10 nm. If the protrusion is larger than about 10 nm, the width D of the land may not be greater than 0.3 xcexcm.
Further, it is also preferable that the ratio D/C of the width D of the land at the top portion thereof to the width C of the land at the bottom portion thereof is within the range of about 0.55-about 0.9. The bottom width C represents a length of the land measured at the bottom thereof along the radial direction(shown in FIG. 4). If the ratio, top width D/bottom width C, is less than about 0.55, push-pull values or the degree of modulation becomes small, and therefore, recording and reproduction of the optical information may not be performed properly. Also, if the ratio D/C is larger than about 0.90, the top width D may not be properly secured due to deteriorated transferability of a stamper shape occurring during the injection molding process of the light transmitting substrate.
In addition, it is also preferable that the thermal conductivity p of the recording layer is within the range of about 0.15xe2x89xa6pxe2x89xa60.25 W/mK. If the thermal conductivity of the recording layer is less than about 0.15 W/mK, a jitter becomes large. On the other hand, if the thermal conductivity of the recording layer is greater than about 0.25 W/mK, a proper degree of modulation may not be obtained. The recording layer is made out of an organic dye containing therein a light stabilizer; and the thermal conductivity of the recording layer depends on the materials included in the dye solution, a composition thereof and the thickness of the recording layer itself and can be varied by changing the dependence on them. A preferable dye includes a cyanine dye, a phthalocyanine dye, an azo dye, a polymethine dye, a triarylmethane dye, a pyrylium dye, a phenanthrene dye, a tetrahydrocholine dye, a triarylamine dye, a croconic methine dye, a merocyanine dye and the like or their mixture, although not limited thereto.
It is preferable that a leveling index L(=1xe2x88x92B/A) is within the range of about 0.2xe2x89xa6Lxe2x89xa60.5. Herein, the leveling index L represents a degree of flatness of the recording layer; A, a depth of the tracking groove; and B, a depth of the recording layer on the groove(shown in FIG. 4). Therefore, as the leveling is more pronounced, the difference between the depth A of the pre-groove and the depth B of the recording layer becomes greater, and thus the value of L increases. If L is equal to zero, which is practically impossible, it means that the recording layer is not leveled at all and thus the depth A of the pre-groove is identical to the depth B of the recording layer. On the other hand, the value of L=1 means that the recording layer is perfectly leveled and thus the depth B of the recording layer is zero. If the leveling index L is within the range above, the thermal conductivity p of the recording layer can be easily controlled to be within the range of about 0.15 W/mKxe2x89xa6pxe2x89xa60.25 W/mK. If L is less than about 0.2, the thermal conductivity becomes large, and the proper degree of modulation may not be obtained. If L is greater than about 0.5, the thermal conductivity decreases, which in turn deteriorates the jitter.